Carbon nanotubes (CNT) possess unique electrical properties, being ideal candidates for various components in modern electronics. Depending on the crystal structure, CNTs have a dual electrical property: semiconducting and metallic.
Single-walled carbon nanotubes (SWNTs) are one particular type of CNTs, which may be used interchangeably in this specification with “CNTs” in the context that is definite and understood by people skilled in the art. SWNTs possess unique optical properties as a result of their one-dimensional nature. Sharp peaks in the density of states, called van Hove singularities (VHS), arise from a quantization of the electronic wave vector in the 1-D system. The SWNTs have a wide range of direct bandgaps matching the solar spectrum, and show strong photoabsorption from infrared to ultraviolet, and exhibit high carrier mobility and reduced carrier transport scattering. These superior properties of SWNTs have made them potential ideal candidate materials for highly active photovoltaic devices, in which semiconductor heterostructures are widely utilized to harvest light energy in connection with solar cells. The photovoltaic effect can be achieved in ideal carbon nanotube diodes. Individual CNTs can form ideal p-n junction diodes. Under illumination, single wall carbon nanotube (SWNT) diodes demonstrate significant power conversion efficiencies resulting in enhanced properties of an ideal diode.
In organic solar cell applications, CNTs were mainly used as nanoscale fillers into polymer matrix or as transparent electrodes for collecting charge carriers. The high aspect ratios and large surface area of nanotubes could be beneficial to exciton dissociation and charge carrier transport thus improving the power conversion efficiency. The conjugated polymers (e.g., P3HT) produce excitons under illumination, while nanotubes embedded into the polymer matrix only provide more interfacial area for exciton dissociation and charge transport path. However, donor-acceptor type structures consisting of conjugated polymers mixed with nanotubes still suffer low conversion efficiency. Further, phase segregation between nanotubes and polymers, aggregation of nanotubes at higher concentration, and coexistence of metallic and semiconducting nanotubes are major limiting factors to the device fabrication and performance. Additionally, nanotube films used as transparent electrodes have shown negligible improvement of efficiency compared with indium tin oxide (ITO) glass.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.